The GUI configuration (colors, fonts, etc.) is changed in a different
way (GUI ‣ …) and is not covered here.

It is possible to change the value of the option temporarily:

with option1=value1 [,option2=value2] command args...

For example:

info set fitting_method # show the current fitting method
set fitting_method = nelder_mead_simplex # change the method# change the method only for this one fit command
with fitting_method = levenberg_marquardt fit
# and now the default method is back Nelder-Mead# multiple comma-separated options can be given
with fitting_method=levenberg_marquardt, verbosity=quiet fit

The ε value used to test floating-point numbers a and b for equality
(it is well known that due to rounding errors the equality test for two
numbers should have some tolerance, and the tolerance should be tailored
to the application): |a−b| < ε. Default value: 10-12.
You may need to decrease it when working with very small numbers.

Format of numbers printed by the info command. It takes as a value
a format string, the same as sprintf() in the C language.
For example setnumeric_format='%.3f' changes the precision
of numbers to 3 digits after the decimal point. Default value: %g.

on_error

Action performed on error. If the option is set to stop
(default) and the error happens in script, the script is stopped.
Other possible values are nothing (do nothing) and exit
(finish program – ensures that no error can be overlooked).

pseudo_random_seed

Some fitting methods and functions, such as
randnormal in data expressions use a pseudo-random
number generator. In some situations one may want to have repeatable
and predictable results of the fitting, e.g. to make a presentation.
Seed for a new sequence of pseudo-random numbers can be set using the
option pseudo_random_seed. If it
is set to 0, the seed is based on the current time and a sequence of
pseudo-random numbers is different each time.

refresh_period

During time-consuming computations (like fitting) user interface can
remain not changed for this time (in seconds).
This option was introduced, because on one hand frequent refreshing of
the program’s window notably slows down fitting, and on the other hand
irresponsive program is a frustrating experience.

xrange and yrange has syntax [min:max]. If the boundaries
are skipped, they are automatically determined using the given datasets.

In the GUI

there is hardly ever a need to use this command directly.

The CLI version on Unix systems visualizes the data using the gnuplot
program, which has similar syntax for the plot range.

Examples:

plot [20.4:50] [10:20] # show x from 20.4 to 50 and y from 10 to 20
plot [20.4:] # x from 20.4 to the end,# y range will be adjusted to encompass all data
plot # all data will be shown

The values of the options autoplot and fit_replot
change the automatic plotting behaviour. By default, the plot is
refreshed automatically after changing the data or the model (autoplot=1).
It is also possible to replot the model when fitting, to show the progress
(see the options fit_replot and refresh_period).

Redirecting the plot command to a file saves a plot as an image:

plot [20.4:50] [10:20] > myplot.png

For now, it works only in fityk (not cfityk) and is less flexible than
Session ‣ Save as Image.

The other valid arguments are filename and title.
They are useful for listing the same values for multiple datasets, e.g.:

=-> @*: print filename, F[0].area, F[0].area.error

print can also print a list where each line corresponds to one data point,
as described in the section Exporting Data.

As an exception, printexpression>filename does not work
if the filename is not enclosed in single quotes. That is because the parser
interprets > as a part of the expression.
Just use quotes (print2+3>'tmp.dat').

On startup, the program runs a script from the
$HOME/.fityk/init file (on MS Windows XP:
C:\DocumentsandSettings\USERNAME\.fityk\init).
Following this, the program executes command passed with the --cmd
option, if given, and processes command line arguments:

if the argument starts with =->, the string following =->
is regarded as a command and executed
(otherwise, it is regarded as a filename),

if the filename has extension “.fit” or the file begins with a “# Fityk”
string, it is assumed to be a script and is executed,

otherwise, it is assumed to be a data file;
columns and data blocks can be specified in the normal way,
see Loading Data.

There are also other parameters to the CLI and GUI versions of the program.
Option “-h” (“/h” on MS Windows) gives the full listing: